Hi,
I just updated to Calibre 5.16 64-bits on Windows 10, but the issue started with version 5.15.
When I edit a book adding
<p class="pagebreak"></p> in the page layout and
.pagebreak {
page-break-after: always;
} in the stylesheet, the viewer in Calibre 5.16 doesn't add a page break.
If I revert back to Calibre 5.14, the viewer adds a page break, so everything is referenced as it should, I guess.
Then, if I install Calibre 5.16, the viewer shows the page break, but if I open the book with the editor and close the book, the viewer doesn't add the page break anymore.
It's really puzzling.

I tried using the new syntax
break-after: always; but it's not working.
I've edited several books this way and it worked fine until now. Any idea? Am I doing something wrong?
Best!